1. Field of the Invention
This invention relates to a semiconductive ceramic composition for a semiconductive ceramic capacitor, and more particularly to a SrTiO.sub.3 -Y.sub.2 O.sub.3 -Nb.sub.2 O.sub.5 system semiconductive ceramic composition suitable for use for a boundary-layer type semiconductive ceramic capacitor and such a capacitor.
2. Background of the Invention
A semiconductive ceramic capacitor which serves as a passive electronic circuit element is generally classified into a surface-layer type one and a boundary-layer type one. The surface-layer type semiconductive ceramic capacitor includes a reduction and reoxidation type semiconductive ceramic capacitor and a barrier-layer type semiconductive ceramic capacitor.
The reduction and reoxidation type semiconductive ceramic capacitor is generally prepared according to the following procedures. A BaTiO.sub.3 or SrTiO.sub.3 system compact having an additive for semiconductivity added thereto is burned or fired in an atmosphere to prepare dielectric ceramic, which is then subjected to a heat treatment in a reducing atmosphere to produce a semiconductive ceramic body. The so-produced semiconductive ceramic body is subjected to a heat treatment in an atmosphere or oxygen atmosphere, so that oxygen may be diffused into the ceramic body through a surface thereof to fill oxygen defects. This results in a composite ceramic body being formed wherein its surface layer acts as a dielectric layer (reoxidation layer) and its interior serves as a semiconductor. Thereafter, electrodes are arranged on both surfaces of the composite ceramic body to provide a small-sized semiconductive ceramic capacitor of large capacity which has electrostatic capacity determined depending on a thickness of its surface layer and is capable of increasing rated voltage by an increase in thickness.
Now, preparation of the barrier-layer type semiconductive capacitor will be described.
A compact which is typically made of a BaTiO.sub.3 system material containing an additive for semiconductivity is burned in an atmosphere and a film of metal such as copper or the like is formed on a surface of the burned compact by vapor deposition. Then, an electrode of a material such as silver or the like of which oxide readily forms a P-type semiconductor is applied onto the metal film and then subjected to a heat treatment in an atmosphere to form a barrier layer of about 0.3 to 3.mu. on a surface thereof. This results in a barrier-layer type semiconductive ceramic capacitor in which its surface forms a barrier layer insulator on which an external electrode is arranged and its interior forms a semiconductor. A capacitor of this type is suitable for use as a low voltage and large capacity capacitor, because it has large electrostatic capacity although it is decreased in dielectric strength because of the barrier layer having a very small thickness.
The boundary-layer type semiconductive ceramic capacitor is typically manufactured according to the following procedures.
A BaTiO.sub.3 or SrTiO.sub.3 system compact containing an additive for semiconductivity is subjected to burning in a reducing atmosphere to prepare a semiconductive ceramic body. Then, metal oxide such as Bi.sub.2 O is applied onto a surface of the ceramic body and then subjected to a heat treatment in an atmosphere, resulting in metal ion penetrating into an interior of the ceramic body to form an insulation layer containing the metal ion at a grain boundary of the ceramic body. An interior of each of the crystal grains of the ceramic forms a valency-controlled semiconductor doped with the additive for conductivity. Thus, an interior of each of grain layers in the ceramic body is changed to the insulation layer which surrounds the valency-controlled semiconductor. The so-formed insulation grain boundary layers are connected together in a matrix-like shape in all directions to form a sponge-like dielectric. Thereafter, electrodes are baked to a boundary-layer type semiconductive ceramic capacitor.
The semiconductive ceramic capacitors described above are limited to use for bypass because they are small-sized and have large capacity but are inferior in voltage characteristics, dielectric loss and frequency characteristics. However, an advance in manufacturing techniques sufficient to improve the characteristics caused a semiconductive ceramic capacitor of which a base material comprises a SrTiO.sub.3 system material to be manufactured which is capable of being extensively used for various purposes extending from coupling, signal circuits and pulse circuits to prevention of noise of a semiconductor.
Nevertheless, the semiconductive ceramic capacitors are still inferior in electrical characteristics as indicated in Table 1 described below, irrespective of such an advance. More particularly, the reduction reoxidation type capacitor is decreased in insulation resistance and increased in dielectric loss as compared with the boundary-layer type capacitor. Likewise the barrier-layer type capacitor has a disadvantage of being decreased in dielectric breakdown voltage to a level as low as 60 to 80 V, decreased in insulation resistance and increased in dielectric loss. Such disadvantages are also encountered with the valency-controlled type capacitor.
A base material of each of such surface-layer type semiconductive ceramic capacitors is a SrTiO.sub.3 system, resulting in a thickness of the ceramic body causing the capacitor to fail to exhibit large capacity of Cs.gtoreq.5 nF/mm.sup.2.
The boundary-layer type semiconductive ceramic capacitor is increased in insulation resistance and decreased in dielectric loss as compared with the surface layer type ones, because its base material is a SrTiO.sub.3 system different from BaTiO.sub.3. However, the capacitor has a capacity as low as 3.0 nF/mm.sup.2 and fails to exhibit large capacity of Cs.gtoreq.5 nF/mm.sup.2.
TABLE 1 __________________________________________________________________________ Insulation Electrostatic Dielectric Insulation Breakdown Capacity Loss Resistance Voltage .epsilon.s .times. Eb** Type of SCC* Cs (nF/mm.sup.2) tan .delta. (%) IR (M.OMEGA.) Vb (V) (V/mm) __________________________________________________________________________ Reduction & 2.9 6.1 700 210 6.9 .times. 10.sup.7 Reoxidation Type Barrier Layer 4.2 4.4 10 80 3.8 .times. 10.sup.7 Type (A) Barrier Layer 4.5 5.7 3 58 2.9 .times. 10.sup.7 Type (B) Conventional 3.3 0.4 7,500 190 7.1 .times. 10.sup.7 Boundary Layer Type Boundary 5.3 0.6 6,000 170 10.2 .times. 10.sup.7 Layer Type (present invention) __________________________________________________________________________ *SCC: Semiconductive ceramic capacitor **.epsilon.s: Dielectric constant, Eb: Insulation breakdown voltage per unit thickness Cs and tan .delta. were measured under conditions of 1 kHz and 1 V rms. I was measured at 50 V for 1 min. Vb was measurd at a D.C voltage raising velocity of 30-50 V/sec.
In the surface layer type semiconductive ceramic capacitor, capacity C is not inversely proportional to its thickness, accordingly, dielectric constant s may be obtained according to the following equations. EQU Cs (nF/mm.sup.2)=8.85.times.10.sup.-6 .epsilon..sub.s /t (1) EQU Vb (V)=Eb.multidot.t (2)
Accordingly, EQU .epsilon..sub.s .multidot.Eb (V/mm)=1.13.times.10.sup.5 Cs.multidot.Vb
The product .epsilon..sub.s .multidot.Eb listed on Table 1 was calculated according to the equations described above.
Formation of an electrode of each of the conventional semiconductive ceramic capacitors described above is generally carried out by applying a silver paste consisting of powdered silver, powdered glass and an organic vehicle onto a surface of the ceramic body and then adhering it thereto by baking. Alternatively, it is carried out by electroless plating of nickel.
Formation of the electrode by baking of a silver paste has an advantage of providing not only a ceramic capacitor which exhibits desired electrostatic capacity and dielectric loss tangent but an electrode of sufficient tensile strength and solderability. However, this causes the produced ceramic capacitor to be expensive because silver is noble metal of a high cost. Also, this has another disadvantage that silver is apt to cause metal migration.
The electroless nickel plating is generally carried out by subjecting a surface of a ceramic body to a treatment for rendering the surface roughed using a mixed solution of ammonium fluoride and nitric acid, subjecting the surface to a treatment using a tin chloride solution and a palladium chloride solution and then immersing it in an electroless nickel plating solution to form an electroless nickel deposit on the surface. The plating further includes the steps of applying a resist onto a portion of the nickel deposit on which an electrode is to be formed and immersing the ceramic body in an etching solution such as nitric acid or the like to remove an unnecessary portion of the nickel deposit. Accordingly the ceramic body is damaged or corroded by various kinds of the solutions containing acid and the like during formation of the electrode to cause decomposition of the surface of the ceramic body. Further, leaving of the plating solution and the like on the ceramic body due to a failure in cleaning causes deterioration of the capacity manufactured.